The invention pertains to pyrometers and particularly to optical pyrometers. More particularly, the invention pertains to multi-detector pyrometers for measurement of nonsteady emitters.
Presently available optical pyrometers are based on either one- or two-channel measurement of the radiance of hot-surfaces or hot-gases, whereby this radiance is measured within the selected spectral band with available detectors, whether visible or infrared (IR). However, none take into account the continuous changes in detector sensitivity or filter transmission versus time and versus ambient temperature. Filter-wheel-based approaches do not achieve accurate temperature measurement because of the short measurement time and the generally sequential nature of the signal generation, which is especially detrimental with unsteady radiation sources such as turbulent flames. In addition, they are generally bulky, costly and wear out.
One application for optical pyrometers may be related to turbine engine efficiency and emissions control, which at times will be used below as an illustrative example, without limiting the applicability of the described pyrometer. The need for and the possible benefits resulting from a successful turbine combustor exit gas temperature sensor are noted. Assuming that turbine stator blades can withstand an operating temperature of 2500 degrees Fahrenheit (° F.), i.e., 1644 degrees Kelvin (° K), the non-uniformity of the combustor exit gas temperatures may force that temperature to be set to no more than about 2300° F. (1533° K, i.e., a drop of over 8 percent) in order to prevent unpredictable “hot streaks” to stay below 3300° F. (2088° K). This forced drop in average temperature and the unpredictable occurrence of hot streaks may result in both turbine efficiency losses as well as excess NOx emissions, respectively. Due to unpredictable fuel injection nozzle coking and/or plugging of air inlets, which may limit the effectiveness of even ideal, zero-variability nozzle fabrication capabilities, one solution may be to measure the exit temperature of each combustor, so that appropriate, active fuel/air ratio control can be implemented.
Examples of previous approaches and why they do not meet the needs for a small, fast, rugged, accurate and stable pyrometer that is able to operate in a harsh environment (−40° to 200° Centigrade (C.)) temperature, non-thermostatted) to measure the temperature of non-steady-state sources, are noted. Single-channel pyrometers may not be suitable because the sensitivities and null-offset of all detectors shift as a function of temperature. Measured radiance temperatures may therefore shift accordingly.
Two-channel pyrometers without a filter wheel but with two (double-decker) detectors may be elegant solutions that use one-beam light inlet from the source, followed by either double-decker silicon (Si) detectors, double-decker Si-PbS or by a beam-splitter to engage two separate detectors that are sensitive to different wavelengths. Their output ratio is a measure for source temperature and may be suitable to monitor unsteady sources. But their output temperature error might become unreasonably large as the ambient temperature deviates from calibration conditions, without temperature-dependent offset compensation. In addition, the double-decker version may be very limited in the choice of the two channel wavelengths to those transmitted by the top detector, and the beam-splitter version may not correct for individual variabilities in the sensitivity drift of the two detectors.
Two-channel-one-detector pyrometers with a filter wheel include those where the two wavelengths may be selected freely among commercially available narrow-band pass filters. However, observation of an unsteady source might require that either the filter wheel be turned faster than the source instability (which may reduce detector observation time and the signal-to-noise ratio (S/N)) or slow enough to raise the S/N (which may then increase the un-relation between the two channel signals due to unsteadiness of the source and the error in the temperature resulting from their signal ratio).
Two-channel-two-detector pyrometer with chopper consisting of two detectors with fixed filters and on/off choppers, may eliminate the time- and temperature-dependent drift of the null-offset. However, it may not compensate for the error caused by individual sensitivity drift in the detector(s) inherent in setups with fixed filter-detector pairings.
Available combustion pattern factor (CPF) sensors for each of the fuel atomizers in a gas turbine engine tended to depend on the measurement of first stator blade temperature sensing via pyrometry or deposited film thermocouples (or resistors) or special temperature-dependent phosphorescent films. Whereas this direct approach to sensing the object to be protected from overheating could be commendable, since the service life of the films has often been too short and the application method somewhat too intrusive to be of practical value. Other possible approaches based on: 1) Analyzing the sonic signature of the multi-burner combustor with an array of passive microphones may require a very large computational effort and is still in its infancy; and 2) Suction pyrometry (with or without thermocouples) may only provide limited spatial coverage.